1. Field of the Invention
The present invention relates to an ionizing analysis apparatus which ionizes (in so-called soft ionization) a hardly volatile macromolecule such as biopolymer, protein, sugar chain, DNA, or drug without decomposing it, and then performs mass spectrometry or the like of this macromolecule.
2. Related Background Art
In the conventional ionizing analysis apparatus for performing mass spectrometry or the like of the above-mentioned hardly volatile macromolecule as a sample, such a technique as laser desorption or electro-spray is used for effecting soft ionization of the sample.
In the laser desorption technique, the sample and a matrix which functions to absorb a laser incident thereon and prevent the sample from decomposing are mixed together, and the resulting mixture is coated on a substrate and dried in the atmosphere so as to prepare an ion source. Thus prepared substrate is attached to the ionizing analysis apparatus, which is then evacuated with a vacuum pump. After a vacuum level reaches a predetermined level, the ion source is irradiated with a laser beam, whereby ions of the matrix and sample are evaporated. As they are introduced into a mass spectrometer and detected by an ion detector, a mass spectrum is obtained. This technique overcomes a problem that, when only a hardly volatile macromolecule as the sample is irradiated with a laser, the soft ionization of the sample cannot be achieved due to the decomposition of the macromolecule.
In the following, an ionizing analysis apparatus using the electro-spray technique will be explained with reference to FIG. 26. An electrolytic solution L containing an ion-dissociated sample is supplied to a capillary 1 having an inner diameter of about 100 .mu.m or less. Due to an electric field generated by a high voltage applied to the capillary 1, a tip portion 2 of the electrolytic solution L attains a needle form. Accordingly, the electrolytic solution L is sprayed from the tip portion 2 so as to be emitted into an atmosphere R as an ion droplet 3 having a diameter of about 1 .mu.m. The atmosphere R is differentially evacuated with a vacuum pump (not depicted) while N.sub.2 gas is fed thereto from a predetermined supply port. As shown in FIG. 27, thus emitted ion droplet 3 gradually reduces its volume and surface area as it splits or its solvent evaporates. As the surface area of the ion droplet decreases, ions of the sample or ions of the solvent migrate to the surface of the droplet. When the droplet further reduces the volume such that its radius reaches a predetermined critical level (about 10 nm), the ions are emitted from the droplet (subjected to so-called ion evaporation) due to Coulomb repulsion among the ions in the droplet. As thus emitted ions are introduced into a mass spectrometer 5 via an ion introduction section 4 of the ionizing analysis apparatus and then are detected by an ion detector 6, a mass spectrum is obtained.
In the method using the laser desorption technique, however, the amount of the matrix is as much as 10.sup.6 times that of the sample, and all of the matrix is evaporated, whereby the efficiency at which the ions of the sample are introduced into the mass spectrometer is 10.sup.-6 to 10.sup.-10, which is very low. Also, in order to prepare the ion source, the mixture of the matrix and sample is coated on the substrate once or several times and dried in the atmosphere after each coating step, the substrate is attached to the ionizing analysis apparatus, and then the ionizing analysis apparatus is evacuated, thereby necessitating a long preparatory time before measurement.
In the method using the electro-spray technique, when the concentration of the sample with respect to the solvent is high, the radius of the ion droplet may not decrease to such an extent that it reaches the above-mentioned critical level, whereby ion evaporation may not occur. Also, when a non-organic solvent such as water is used, the mist-like ion droplet 3 may not sufficiently be emitted from the tip of the capillary 1. In order to overcome such a state where spraying is insufficient, the electric field strength may be raised so as to supply a higher energy for spraying. As the electric field strength increases, however, electric discharge tends to occur. Further, in the electro-spray technique, since not only the sample but also all the solvent is evaporated, and the solvent is removed by differential evacuation, the efficiency at which the sample is introduced into the mass spectrometer is also very low, i.e., 10.sup.-6 to 10.sup.-10.
On the other hand, as disclosed in a publication (M. Wilm et al, Anal. chem., 1996, 68, 1-8), the transfer efficiency can be improved in the electro-spray technique when the diameter of the capillary used therein is reduced. In this case, the efficiency at which the ions are transferred to the mass spectrometer can be increased to about 10.sup.-2. The size of droplet that can be emitted from the capillary with such a small diameter has already reached its limit, however, whereby it is difficult to further improve the transfer efficiency according to this method.
Also, FD (field desorption) technique has been known as a method in which a sample is mounted on a needle, dried, and then is inserted in a vacuum atmosphere, to which a voltage of several kV is applied so as to form an electric field for evaporating ions. This method may not be practical, however, since it requires a duration of about one day for operations such as drying of the sample.
In order to eliminate the complicated operations of the FD technique, proposed (in Japanese Patent Application Laid-Open No. 3-285245) is a method in which, while a liquid chromatography effluent is spouted toward a nozzle, a high voltage is applied to the needle carrying the sample, so as to ionize the sample. In this method, however, the sample-ionizing efficiency is not so high, and abnormal electric discharge may be generated, or the medium itself may be ionized so as to generate a background noise.
On the other hand, Japanese Patent Application Laid-Open No. 8-148117 discloses an ionizing analysis apparatus equipped with a sample-material sampling needle for emitting into a chamber a sample material supplied through a tube. This apparatus is disadvantageous in that the sampling accuracy of the sample material supplied through the tube may be insufficient.
Accordingly, it is an object of the present invention to provide an ionizing analysis apparatus which is able to reduce the measurement time since ions can be generated in a short time, measure the ions with a high sensitivity, yield a high efficiency at which the ions are introduced into a mass spectrometer, and operate continuously.